In recent years, regions for development of crude oil, natural gas, and other energy resources have been shifting to the North Sea, Siberia, Northern America, Sakhalin, and other frigid areas and further to the North Sea, Gulf of Mexico, Black Sea, Mediterranean, Indian Ocean, and other deep seas, that is, regions of harsh natural environments. Further, from the viewpoint of the emphasis on prevention of global warming, there has been an increase in development of natural gas. At the same time, from the viewpoint of the economicalness of pipeline systems, reduction of the weight of the steel materials and increase in the operating pressure have been sought. The properties sought from line pipe have become increasingly sophisticated and diverse in accordance with these changes in environmental conditions. They may be roughly classified into demands for (1) greater wall thickness/higher strength, (2) higher toughness, (3) reduction of the carbon equivalent (Ceq) accompanying improvement of on-site weldability (circumferential direction weldability), (4) increased corrosion resistance, and (5) high deformation performance in frozen ground and earthquake/fault line belts. Further, these properties are usually demanded in combination along with the usage environments.
Furthermore, with the backdrop of the recent increase in crude oil and natural gas demand, far off locations and regions of tough natural environments which have been passed over for development due to their unprofitability are starting to be exploited in earnest. In particular, the line pipe used for pipelines transporting crude oil and natural gas over long distances are being strongly required to be increased in thickness and strength for improving the transport efficiency and also to be increased in toughness so as to be able to withstand use in frigid areas. Achievement of both of these demanded properties is becoming a pressing technical issue.
On the other hand, steel pipe for line pipe can be classified by its process of production into seamless steel pipe, UOE steel pipe, seam welded steel pipe, and spiral steel pipe. These are selected according to the application, size, etc., but with the exception of seamless steel pipe, each by nature is made by shaping steel plate or steel strip into a tubular form, then welding the seam to obtain a steel pipe product. Furthermore, these welded steel pipes can be classified according to if they use hot coil or use plate for the materials. The former are seam welded steel pipe and spiral steel pipe, while the latter are UOE steel pipe. For high strength, large diameter, thick wall applications, the latter UOE steel pipe is generally used, but for cost and speed of delivery, the former seam welded steel pipe and spiral steel pipe made using hot coil as a material are being required to be made higher in strength, larger in diameter, and thicker in walls.
In UOE steel pipe, technology for production of high strength steel pipe corresponding to the X120 grade has been disclosed (for example, see “Nippon Steel Monthly”, No. 380, 2004, page 70).
However, the above art is predicated on use of thick-gauge plate as a material. To achieve both higher strength and greater wall thickness, a feature of the thick-gauge plate production process, that is, interrupted direct quench (IDQ), is used at a high cooling rate and low cooling stop temperature. In particular, to secure strength, quench strengthening (texture strengthening) is being used.
As opposed to this, with the hot coil material of seam welded steel pipe and spiral steel pipe covered by the present invention, there is the feature of the coiling process. Due to restrictions in the capacity of coilers, it is difficult to coil a thick-gauge material at a low temperature, so it is impossible to stop the cooling at the low temperature required for quench strengthening. Therefore, securing strength by quench strengthening is difficult.
On the other hand, as technology for achieving both the higher strength and greater wall thickness and the low temperature toughness of hot coil for line pipe, the technology has been disclosed of adding Ca—Si at the time of refining to make the inclusions spherical, adding V with the crystal refinement effect in addition to the strengthening elements of Nb, Ti, Mo, and Ni, and, furthermore, making the microstructure bainitic ferrite or acicular ferrite to secure the strength by combining low temperature rolling and low temperature cooling (for example, see Japanese Patent No. 3846729 (Japanese Patent Publication (A) No. 2005-503483)).
However, the above art does not allude to the problem inherent to hot coil for spiral steel pipe for line pipe, that is, anisotropy of toughness in the rolling direction, width direction, and pipe circumferential direction after formation into spiral steel pipe.
Spiral steel pipe is produced by uncoiling a hot coil while arc welding the seam in a spiral shape. For this reason, the properties in the pipe circumferential direction becoming important after production as line pipe are important. Despite this, the circumferential direction after pipe production and the width direction of the hot coil do not match. In general, the hot coil material rolled at a low temperature as a material for line pipe has anisotropy of properties from the rolling direction. In particular, the tensile strength tends to drop in a direction 45° from the rolling direction. Therefore, improving the strength-toughness balance in this direction 45° from the rolling direction means improving the performance as steel pipe for spiral line pipe.
Specifically, the tensile strength in the pipe circumferential direction after pipemaking satisfies the API-X65 standard or more after pipemaking, so if the tensile strength in that direction of the steel plate is 585 MPa or more and the ductility fracture rate in the direction corresponding to the pipe circumferential direction after pipemaking in a DWTT test at −20° C. is 90% or more of that in the hot coil width direction, the ductility fracture rate in the pipe circumferential direction in a DWTT test at −20° C. after pipemaking becomes 70% or more, and the strength-toughness balance satisfies the characteristics required as spiral steel pipe for line pipe applications. Furthermore, in the above art, regarding the alloy elements, it is necessary to add the extremely expensive alloy element V in a certain amount or more. Due to this, not only is an increase in cost invited, but also the on-site weldability is liable to be reduced. Further, in the method of production, to secure the strength-toughness balance, the coiling temperature has to be lowered. To enable this, sometimes the coiler capacity has to be increased or other special measures taken facility wise.